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Failure analysis of ferritic stainless steel locking ring
Materials and Design 30 (2009) 4454–4458
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Failure analysis of ferritic stainless steel locking ring A. Nusair Khan *, I. Salam Metallurgy Division, Rawalpindi, Pakistan
a r t i c l e
i n f o
Article history: Received 16 January 2009 Accepted 20 March 2009 Available online 26 March 2009
a b s t r a c t A locking ring made of Ferritic stainless steel, fitted in a pressure gauge, failed after the service of seven years. The mode of failure was analyzed by using Scanning Electron Microscope and optical microscope. A finite element model proposed in this work demonstrated the stress distribution in different regions of the locking ring. It was observed that the ring failed in the same region where the maximum stresses were predicted. Furthermore, a detailed study on the mode of failure found that the failure occurred in stress corrosion cracking conditions. Ó 2009 Published by Elsevier Ltd.
1. Introduction The simplest stainless steels contain only iron and chromium. Chromium is a ferrite stabilizer, therefore, the stability of the ferritic structure increases with chromium content. The corrosion and oxidation resistances of the ferritic steels are directly related to their chromium contents. The low chromium grades, AISI 405 and 409, are used where moderate corrosion resistance is required. Whereas the 17% chromium steels, AISI 430, 434 and 436, have good corrosion resistance to atmospheric conditions. Chromium and carbon in ferritic stainless steels react to form carbide. These precipitates are formed both at grain boundaries and within the grains which lead to chromium depletion in the surrounding areas. The depletion of Cr leaves the steel in a sensitized condition, further, the toughness is also reduced by carbide precipitation. Similarly, when these steels are heated between 370 and 480 °C, precipitation of alpha prime, a body-centered cubic, Cr-rich phase, also occurs and reduces the overall toughness. The time required for precipitation of alpha prime is much longer than the time required for sensitization; therefore, alpha-prime precipitation is generally associated with in-service exposure, while sensitization may develop during short-term exposure, such sensitization makes ferritic Cr alloys susceptible to intergranular corrosion and stress corrosion cracking. This effect is similar to what occurs in standard 18Cr–8Ni grades of austenitic, face-centered cubic stainless steel. However, carbide precipitation occurs much faster in ferritic stainless steel, due to the higher carbon contents and much lower solubility of carbon in its bcc atomic structure. Ferrite stainless steels are highly resistant and are in some cases immune to chloride stress corrosion cracking. These grades are frequently considered for thermal transfer applications. This is one of
* Corresponding author. E-mail address: [email protected] (A.N. Khan). 0261-3069/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.matdes.2009.03.027
the reasons that the same type of steel is utilized in this study of locking ring. The aim of this study is to find the reasons that lead to the cause of failure in the subject, locking ring. The following nomenclature will be utilized in the rest of the document for discussion purpose. Fig. 1 illustrates the two sides/ surfaces of the locking ring. Inner side/surface: refers the inner circumference of the locking ring. Outer side/surface: refers the outer circumference of the locking ring. 2. History A fractured locking ring was received for failure analysis purpose. The ring was employed in a pressure gauge, shown in Fig. 2. The whole assembly of the gauge was fitted at one of the ports of the high temperature furnace. This gauge was under use for the last seven years and experienced environmental changes (dirt, moisture, oxides, etc.) during pumping in the furnace chamber. On complaint of gauge problems (not reading the exact values), the gauge was dismantled from the furnace. On disassembling the ring from the gauge, the ring was found fractured at different locations, which can be seen in Fig. 1. 3. Visual inspection There were five pieces of the same locking ring, fractured at different positions Fig. 1. These pieces were examined under a stereomicroscope for general fracture studies. It was observed that the fracture surface as well as the other surface had a layer of rust, shown in Fig. 3. Since this gauge was fitted in a furnace for the last seven years it was expected that many contaminations might be deposited on the surface of the gauge. In general, the fracture surface can be divided into three distinct regions, i.e. a region having a layer of rust, region without rust, and a shear lip region Fig. 3.
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A.N. Khan, I. Salam / Materials and Design 30 (2009) 4454–4458 Table 1 Chemical composition of the fractured locking ring. Elements
Fe Cr Mo Si Mn C S
Weight percentage Fractured sample
Nearest standard AISI 434
Bal. 16.5 1.15 0.4 0.6 – –
Bal 16–18 0.75–1.25 1.0 max. 1.0 max. 0.12 max. 0.03
Fig. 1. Fractured locking ring in as-received condition.
4. Chemical composition The chemical composition of the fractured ring was determined by an Energy Dispersive Spectrometer (EDS) attached with a Scanning Electron Microscope (SEM). However, carbon/sulphur analysis was not conducted due to lack of quantity of the sample. Neverthe-
Fig. 4. Microstructure of the locking ring.
less, it is observed that the ring is made of ferritic stainless steel of grade AISI 434; the detailed weight percentage is given in Table 1. Ferritic stainless steel of type AISI 434 is the modified form of AISI 430 stainless steels with 1%Mo addition and is more corrosion resistant [1,2]. 5. Optical microscopy
Fig. 2. Locking ring holding the internal parts of the gauge.
A region close to the fractured surface was selected and sectioned from the ring. The cross section was then mounted and ground for the optical microscopy. In the as-polished condition no crack was observed in the cross section of the sample. The samples were etched in Kalling for a few seconds and the grains of ferrite were observed in the sample. Fig. 4 shows the microstructure of the sample.
Fig. 3. Fractured surface, under the stereo-microscope, revealing different features.
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A.N. Khan, I. Salam / Materials and Design 30 (2009) 4454–4458
Fig. 5. Polished surface showing the pits and the crack initiation sites.
Fig. 6. Corrosion attack on the grain boundaries (arrows) at the outer surface of the ring.
6. Electron microscopy The sample prepared for optical microscopy was also used in electron microscope study. A network of cracks (arrows) initiating from the surface of the locking ring was observed, demonstrated in Fig. 5. Moreover, corrosion attack on the grain boundaries near the surface was also evident in the as-polished condition of the sample, these cracks act as crack initiation sites as is evident in Fig. 6. Before the fracture surfaces were studied on SEM, the surfaces were cleaned ultrasonically from rust and other debris. In general, the fracture seemed to be initiated from the outer side of the locking ring, Fig. 7, marked by the flow pattern. The overall fracture mode was intergranular on the outer side of the ring, Fig. 8a, whereas the fracture surface observed on the inner side was a mixture of intergranular and transgranular modes, Fig. 8b. High concentration of chromium carbides was also observed in the outer region of the fracture surface, Fig. 8a. Further, on scanning the other regions of the locking ring, i.e. apart from the fracture surface, a network of cracks was observed at different locations, Fig. 9.
Fig. 7. General fracture surface showing the initiation region of the crack (encircled).
A.N. Khan, I. Salam / Materials and Design 30 (2009) 4454–4458
7. Finite element modeling Finite element model (FEM) was used to predict the stress distribution in the locking ring. For this purpose ANSYS structural software [3] was utilized. The geometry was created in ANSYS preProcessor using actual dimension of the locking ring. Two dimensional finite element analysis was conducted using four-noded
4457
quadrilateral elements (PLANE42) under plane-stress conditions. The half model was used in FEA due to symmetry in loading and the geometry of the ring. During Finite Element Analysis, an isotropic material with modulus of elasticity E = 200 GPa and Poisson’s ratio y = 0.3 was used [4]. The meshed model is shown in Fig. 10. The stress was applied from the inner side of the ring and the outer side was constrained to simulate the actual loading condi-
Fig. 8. (a) Intergranular fracture surface observed in the outer side of the ring. Dispersed chromium carbides also observed on the surface. (b) Transgranular fracture surface observed on the inner side of the ring.
Fig. 9. Surface crack on the surface of the locking ring.
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A.N. Khan, I. Salam / Materials and Design 30 (2009) 4454–4458
Fig. 10. The meshed model of the half locking ring.
fractured at five different regions. Five fractures of the locking ring refer the presence of cracks which might be propagated under the external stress condition. On observing the internal surfaces and the cross sections of the sample it was confirmed that the cracks were present on the external surface of the ring. The presence of cracks on the outer surface of the ring can be expected, since this portion was under tension as compared to the inner side of the ring. Finite element analysis demonstrated that the maximum stressed region was present close to both the free ends of the ring. Practically, on observing the fractured ring it can be noticed that the two fractures are exactly from the same area where the FEA indicates the maximum level of stresses. This not only validates the FEA model but also confirms that the stresses played an important role in the failure of the ring. SEM examination confirms the intergranular mode of the fracture near the outer surface (crack initiation region). The nature of the fracture keeping in view the presence of external surface cracks and tensile mode of stresses leads to the hypothesis that the sample might be fractured in stress corrosion conditions. The mechanism for intergranular corrosion in ferritic stainless steels is largely accepted as the precipitation of chromium compounds at grain boundaries, and this causes chromium depletion in the grains immediately adjacent to the boundaries. SEM examination also reveals the presence of high concentration of chromium carbides (Fig. 8a). The presence of these carbides consequently deplete the nearby area in chromium contents. This lowering of the chromium content leads to increased corrosion rates in the oxidizing environment. Susceptibility to intergranular corrosion may mean susceptibility to intergranular stress corrosion cracking (SCC). The stresses required to cause SCC are small, usually below the macroscopic yield stress, and are tensile in nature. Therefore, the tensile stresses on the outer side of the locking ring, as discussed above, may lead to SCC. Further, the transgranular features observed near the inner side were caused due to the force exercised during the unlocking of the ring from the main assembly. 9. Conclusion The fractured locking ring was under stressed condition during its service. The outer side of the ring was under tensile stresses. This was calculated by Finite element modeling. These stresses and favorable corrosion conditions initiate the stress corrosion cracking in the ring, which leads to final fracture of the locking ring.
Fig. 11. Comparison of original fractured ring and simulated half locking ring. (a) Part of original locking ring. (b) Contour plot of the stress distribution predicted by ANSYS structural software.
tions. After applying the boundary conditions and getting the solution the contour plots were obtained and are presented in Fig. 11b. The maximum stressed region was close to the end of the rings. For comparison the original fracture surface is also shown in Fig. 11a, with simulated locking ring. 8. Discussion Five pieces of the failed locking ring were received. On applying load, for disengaging the ring from the main assembly, the ring
References [1] Lula RA, editor. Stainless steel. Metals Park, Ohio: ASM; 1986. p. 52–4. [2] J.R. Davis (Ed.), Alloying understanding – the basics. Materials Park, OH : Davis and Associates, ASM International; 2006. p. 257–86. [3] ANSYS Inc. ANSYS elements manual, 7th edition, 2004. [4] ASM metals hand book (Digital version), vol. 1, Metals Park, OH. p. 978.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Technical Report
Failure analysis of ferritic stainless steel locking ring A. Nusair Khan *, I. Salam Metallurgy Division, Rawalpindi, Pakistan
a r t i c l e
i n f o
Article history: Received 16 January 2009 Accepted 20 March 2009 Available online 26 March 2009
a b s t r a c t A locking ring made of Ferritic stainless steel, fitted in a pressure gauge, failed after the service of seven years. The mode of failure was analyzed by using Scanning Electron Microscope and optical microscope. A finite element model proposed in this work demonstrated the stress distribution in different regions of the locking ring. It was observed that the ring failed in the same region where the maximum stresses were predicted. Furthermore, a detailed study on the mode of failure found that the failure occurred in stress corrosion cracking conditions. Ó 2009 Published by Elsevier Ltd.
1. Introduction The simplest stainless steels contain only iron and chromium. Chromium is a ferrite stabilizer, therefore, the stability of the ferritic structure increases with chromium content. The corrosion and oxidation resistances of the ferritic steels are directly related to their chromium contents. The low chromium grades, AISI 405 and 409, are used where moderate corrosion resistance is required. Whereas the 17% chromium steels, AISI 430, 434 and 436, have good corrosion resistance to atmospheric conditions. Chromium and carbon in ferritic stainless steels react to form carbide. These precipitates are formed both at grain boundaries and within the grains which lead to chromium depletion in the surrounding areas. The depletion of Cr leaves the steel in a sensitized condition, further, the toughness is also reduced by carbide precipitation. Similarly, when these steels are heated between 370 and 480 °C, precipitation of alpha prime, a body-centered cubic, Cr-rich phase, also occurs and reduces the overall toughness. The time required for precipitation of alpha prime is much longer than the time required for sensitization; therefore, alpha-prime precipitation is generally associated with in-service exposure, while sensitization may develop during short-term exposure, such sensitization makes ferritic Cr alloys susceptible to intergranular corrosion and stress corrosion cracking. This effect is similar to what occurs in standard 18Cr–8Ni grades of austenitic, face-centered cubic stainless steel. However, carbide precipitation occurs much faster in ferritic stainless steel, due to the higher carbon contents and much lower solubility of carbon in its bcc atomic structure. Ferrite stainless steels are highly resistant and are in some cases immune to chloride stress corrosion cracking. These grades are frequently considered for thermal transfer applications. This is one of
* Corresponding author. E-mail address: [email protected] (A.N. Khan). 0261-3069/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.matdes.2009.03.027
the reasons that the same type of steel is utilized in this study of locking ring. The aim of this study is to find the reasons that lead to the cause of failure in the subject, locking ring. The following nomenclature will be utilized in the rest of the document for discussion purpose. Fig. 1 illustrates the two sides/ surfaces of the locking ring. Inner side/surface: refers the inner circumference of the locking ring. Outer side/surface: refers the outer circumference of the locking ring. 2. History A fractured locking ring was received for failure analysis purpose. The ring was employed in a pressure gauge, shown in Fig. 2. The whole assembly of the gauge was fitted at one of the ports of the high temperature furnace. This gauge was under use for the last seven years and experienced environmental changes (dirt, moisture, oxides, etc.) during pumping in the furnace chamber. On complaint of gauge problems (not reading the exact values), the gauge was dismantled from the furnace. On disassembling the ring from the gauge, the ring was found fractured at different locations, which can be seen in Fig. 1. 3. Visual inspection There were five pieces of the same locking ring, fractured at different positions Fig. 1. These pieces were examined under a stereomicroscope for general fracture studies. It was observed that the fracture surface as well as the other surface had a layer of rust, shown in Fig. 3. Since this gauge was fitted in a furnace for the last seven years it was expected that many contaminations might be deposited on the surface of the gauge. In general, the fracture surface can be divided into three distinct regions, i.e. a region having a layer of rust, region without rust, and a shear lip region Fig. 3.
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A.N. Khan, I. Salam / Materials and Design 30 (2009) 4454–4458 Table 1 Chemical composition of the fractured locking ring. Elements
Fe Cr Mo Si Mn C S
Weight percentage Fractured sample
Nearest standard AISI 434
Bal. 16.5 1.15 0.4 0.6 – –
Bal 16–18 0.75–1.25 1.0 max. 1.0 max. 0.12 max. 0.03
Fig. 1. Fractured locking ring in as-received condition.
4. Chemical composition The chemical composition of the fractured ring was determined by an Energy Dispersive Spectrometer (EDS) attached with a Scanning Electron Microscope (SEM). However, carbon/sulphur analysis was not conducted due to lack of quantity of the sample. Neverthe-
Fig. 4. Microstructure of the locking ring.
less, it is observed that the ring is made of ferritic stainless steel of grade AISI 434; the detailed weight percentage is given in Table 1. Ferritic stainless steel of type AISI 434 is the modified form of AISI 430 stainless steels with 1%Mo addition and is more corrosion resistant [1,2]. 5. Optical microscopy
Fig. 2. Locking ring holding the internal parts of the gauge.
A region close to the fractured surface was selected and sectioned from the ring. The cross section was then mounted and ground for the optical microscopy. In the as-polished condition no crack was observed in the cross section of the sample. The samples were etched in Kalling for a few seconds and the grains of ferrite were observed in the sample. Fig. 4 shows the microstructure of the sample.
Fig. 3. Fractured surface, under the stereo-microscope, revealing different features.
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A.N. Khan, I. Salam / Materials and Design 30 (2009) 4454–4458
Fig. 5. Polished surface showing the pits and the crack initiation sites.
Fig. 6. Corrosion attack on the grain boundaries (arrows) at the outer surface of the ring.
6. Electron microscopy The sample prepared for optical microscopy was also used in electron microscope study. A network of cracks (arrows) initiating from the surface of the locking ring was observed, demonstrated in Fig. 5. Moreover, corrosion attack on the grain boundaries near the surface was also evident in the as-polished condition of the sample, these cracks act as crack initiation sites as is evident in Fig. 6. Before the fracture surfaces were studied on SEM, the surfaces were cleaned ultrasonically from rust and other debris. In general, the fracture seemed to be initiated from the outer side of the locking ring, Fig. 7, marked by the flow pattern. The overall fracture mode was intergranular on the outer side of the ring, Fig. 8a, whereas the fracture surface observed on the inner side was a mixture of intergranular and transgranular modes, Fig. 8b. High concentration of chromium carbides was also observed in the outer region of the fracture surface, Fig. 8a. Further, on scanning the other regions of the locking ring, i.e. apart from the fracture surface, a network of cracks was observed at different locations, Fig. 9.
Fig. 7. General fracture surface showing the initiation region of the crack (encircled).
A.N. Khan, I. Salam / Materials and Design 30 (2009) 4454–4458
7. Finite element modeling Finite element model (FEM) was used to predict the stress distribution in the locking ring. For this purpose ANSYS structural software [3] was utilized. The geometry was created in ANSYS preProcessor using actual dimension of the locking ring. Two dimensional finite element analysis was conducted using four-noded
4457
quadrilateral elements (PLANE42) under plane-stress conditions. The half model was used in FEA due to symmetry in loading and the geometry of the ring. During Finite Element Analysis, an isotropic material with modulus of elasticity E = 200 GPa and Poisson’s ratio y = 0.3 was used [4]. The meshed model is shown in Fig. 10. The stress was applied from the inner side of the ring and the outer side was constrained to simulate the actual loading condi-
Fig. 8. (a) Intergranular fracture surface observed in the outer side of the ring. Dispersed chromium carbides also observed on the surface. (b) Transgranular fracture surface observed on the inner side of the ring.
Fig. 9. Surface crack on the surface of the locking ring.
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A.N. Khan, I. Salam / Materials and Design 30 (2009) 4454–4458
Fig. 10. The meshed model of the half locking ring.
fractured at five different regions. Five fractures of the locking ring refer the presence of cracks which might be propagated under the external stress condition. On observing the internal surfaces and the cross sections of the sample it was confirmed that the cracks were present on the external surface of the ring. The presence of cracks on the outer surface of the ring can be expected, since this portion was under tension as compared to the inner side of the ring. Finite element analysis demonstrated that the maximum stressed region was present close to both the free ends of the ring. Practically, on observing the fractured ring it can be noticed that the two fractures are exactly from the same area where the FEA indicates the maximum level of stresses. This not only validates the FEA model but also confirms that the stresses played an important role in the failure of the ring. SEM examination confirms the intergranular mode of the fracture near the outer surface (crack initiation region). The nature of the fracture keeping in view the presence of external surface cracks and tensile mode of stresses leads to the hypothesis that the sample might be fractured in stress corrosion conditions. The mechanism for intergranular corrosion in ferritic stainless steels is largely accepted as the precipitation of chromium compounds at grain boundaries, and this causes chromium depletion in the grains immediately adjacent to the boundaries. SEM examination also reveals the presence of high concentration of chromium carbides (Fig. 8a). The presence of these carbides consequently deplete the nearby area in chromium contents. This lowering of the chromium content leads to increased corrosion rates in the oxidizing environment. Susceptibility to intergranular corrosion may mean susceptibility to intergranular stress corrosion cracking (SCC). The stresses required to cause SCC are small, usually below the macroscopic yield stress, and are tensile in nature. Therefore, the tensile stresses on the outer side of the locking ring, as discussed above, may lead to SCC. Further, the transgranular features observed near the inner side were caused due to the force exercised during the unlocking of the ring from the main assembly. 9. Conclusion The fractured locking ring was under stressed condition during its service. The outer side of the ring was under tensile stresses. This was calculated by Finite element modeling. These stresses and favorable corrosion conditions initiate the stress corrosion cracking in the ring, which leads to final fracture of the locking ring.
Fig. 11. Comparison of original fractured ring and simulated half locking ring. (a) Part of original locking ring. (b) Contour plot of the stress distribution predicted by ANSYS structural software.
tions. After applying the boundary conditions and getting the solution the contour plots were obtained and are presented in Fig. 11b. The maximum stressed region was close to the end of the rings. For comparison the original fracture surface is also shown in Fig. 11a, with simulated locking ring. 8. Discussion Five pieces of the failed locking ring were received. On applying load, for disengaging the ring from the main assembly, the ring
References [1] Lula RA, editor. Stainless steel. Metals Park, Ohio: ASM; 1986. p. 52–4. [2] J.R. Davis (Ed.), Alloying understanding – the basics. Materials Park, OH : Davis and Associates, ASM International; 2006. p. 257–86. [3] ANSYS Inc. ANSYS elements manual, 7th edition, 2004. [4] ASM metals hand book (Digital version), vol. 1, Metals Park, OH. p. 978.